In recent years, information communications have been advanced with the progress of computer technology as a background. In an automobile, a large number of control units are mounted for respective devices. A network system is provided in such a manner that the control units are set as nodes and connected by communication lines. Thus, data communications can be performed with one another among the nodes.
In one network system, a supervisory device is disposed as one of the nodes. This supervisory device monitors or supervises data frames transmitted from the respective control units and collects information expressive of the operating states (abnormalities) of the control units on the basis of the data frames. Thus, when any abnormality has occurred in the system, the control unit undergoing the abnormality can be specified from the collected information.
JP-A-11-341572, for example, discloses a multiplex data transmission system. In this system, a check memory which has a number of bits equal to the number of nodes is circulated through a network. Each node having received the check memory sets a flag at the bit assigned to the particular node, and transmits the check memory to the node of the next address. In a case where the particular node does nor receive an acknowledgment frame responsive to the transmitted check memory from the node of the next address, it transmits the check memory to the node of the next but one address. Thus, on the supervisory device side, any abnormal node can be specified from the flags of the check memory having been circulated through the network.
Besides, JP-A-11-332086, for example, discloses an abnormality detection method. In this method, a communication line is juxtaposed with a power source line so that the communication line may be electromagnetically influenced when the power source is abnormal. Thus, a communication abnormality is intentionally incurred upon the occurrence of the power source abnormality. On a supervisory device side, the abnormality of the power source line can be detected by supervising the frequency of occurrence of the communication abnormalities.
In the former system, when any abnormality has occurred in the system, the node undergoing the abnormality can be detected. In the latter method, when the abnormality has occurred in the power source, the occurrence of the abnormality can be detected. However, neither of the related techniques can specify the cause of the abnormality.
Therefore, in a case where any specified control unit continues to operate due to a malfunction in spite of a state in which the controlled device has stopped its operation, so that all the control units constituting the network system ought to stop their operations, or in a case where any control unit repeats starts and stop due to a malfunction, although the malfunctioning control unit itself can be specified by applying the former system, a long time is expended in specifying the cause of the malfunction and then improving the system.
In this case, especially when the abnormality cause of the control unit is a very infrequent abnormality cause which arises in a single-shot manner under exceptional conditions, the abnormality cause cannot be specified and hence the system cannot be improved by specifying the abnormality cause.
In order to solve this problem, it is proposed to record control contents in every control unit beforehand and to specify the abnormality cause from the stored contents. However, the quantity of data to be stored becomes enormous in a system whose control is complicated as in the automobile. Further, it is difficult to specify the abnormality cause from among the data of the enormous control contents in a short time, so that such a method cannot be adopted.